Circumsporozoite (CS) proteins have been shown to act as the protective antigens capable of inducing sporozoite-specific immunity in several species of malaria. Monoclonal antibodies raised against surface antigens derived from specific geographic isolates of Plasmodium falciparum, P. vivax and P. berghei react with other isolates of the same species. However, monoclonal antibodies raised against sporozoites of the simian malaria, P. cynomolgi, from one area do not react with sporozoites of other geographic isolates of that species. Analysis of the organization of the DNA within and surrounding certain genes of these P. cynomolgi isolates show significant differences indicative of at least incipient speciation. P. vivax appears to be a member of the P. cynomolgi complex of species.